The present invention relates to a scanning optical system used for a laser beam printer, a laser facsimile machine, or the like. In particular, the present invention relates to a scanning optical system including first and second anamorphic optical elements to converge a light beam deflected by a deflector onto an object surface.
An example of this type of scanning optical system is disclosed in Japanese Laid Open Patent Publication No. HEI 8-68957. In the disclosed scanning optical system, a laser beam emitted from a semiconductor laser is converged by a cylindrical lens to form a linear image in the vicinity of a polygon mirror. The laser beam is deflected and scanned in a main scanning direction by the polygon mirror and is then converged by an imaging optical system including a curved surface mirror and an anamorphic lens, to form a locus of beam spot on the object surface.
The polygon mirror and the curved surface mirror are arranged such that incident and reflected laser beams are separated in an auxiliary scanning direction.
In the main scanning direction, the laser beam incident to the curved surface mirror is essentially a parallel beam and is converged by means of the positive power of the curved surface mirror onto the object surface. Further, in the auxiliary scanning direction, the laser beam incident to the curved surface mirror is strongly divergent and is converted into a weakly convergent beam by the curved surface mirror and then, is converged by the anamorphic lens onto the object surface.
Since the disclosed scanning optical system is designed for a relatively large optical system in which a distance between the polygon mirror and the curved surface mirror, a distance between the curved surface mirror and the anamorphic lens, and a size of the curved surface mirror itself are relatively large, the optical system cannot be easily made compact.
In particular, if the distance from the polygon mirror to the anamorphic lens via the curved surface mirror decreases in order to reduce the size, differences between the optical path length and incident angle to the anamorphic lens of a light beam directed to the center in the main scanning direction and a light beam directed to the periphery will be enlarged, since the scanning angle range must increase while keeping the scanning width in the main scanning direction constant.
As mentioned above, since the reflected light from the curved surface mirror is divergent light in the auxiliary scanning direction, the variation of the optical path length of the incident light beam to the anamorphic lens in accordance with the change of the deflecting angle changes the diameter of the incident light beam in the auxiliary scanning direction.
The variation of the diameter of the light beam in accordance with the change of the deflecting angle changes an effective F-number of the light beam, and thus the spot diameter on the object surface in the auxiliary scanning direction varies in accordance with the distance from the center of the scanning angle and resolution of the pattern formed on the object surface cannot be kept constant.
Additionally, the change of the effective F-number is considered as a variation of the angular magnification of the optical system from the cylindrical lens to the object surface. That is, although the convergent angle of the light beam exiting from the cylindrical lens is constant, the convergent angle of the light beam exiting from the anamorphic lens varies according to the scanning position of the beam spot, and thus, the angular magnification varies according to the scanning position of the beam spot.
In a multi-beam scanning optical system, which scans a plurality of light beams simultaneously to form a plurality of scanning lines on the object surface as locuses of beam spots per scan, the variation of the angular magnification causes a variation of the distance between the scanning lines in the auxiliary scanning direction in accordance with the scanning position of the beam spots. That is, at least one of the scanning lines is curved like a bow. In this specification, the variation of the distance between the scanning lines is defined as xe2x80x9ca differential bowxe2x80x9d. If the differential bow is too large the performance of the scanning optical system is reduced.
It is therefore an object of the present invention to provide a scanning optical system that can reduce a variation of the spot diameter in the auxiliary scanning direction according to the scanning position and can reduce differential bow in a multi-beam scanning optical system, even if the distance between a polygon mirror and an anamorphic lens is reduced in order to reduce the size of the scanning optical system.
According to an aspect of a scanning optical system according to the present invention, which is provided with an imaging optical system having first and second anamorphic optical elements to converge a deflected light from a deflector onto an object surface, a power of the first anamorphic optical element in the auxiliary scanning direction varies according to the position in the main scanning direction such that a magnification of the imaging optical system in the auxiliary scanning direction holds constant in any angle of scanning.
With the above construction, when the exit light beam from the first anamorphic optical element is divergent in the auxiliary scanning direction, the degree of the divergence can be controlled in accordance with the position in the main scanning direction to keep the diameter of the light beam in the auxiliary scanning direction constant when the light beam enters into the second anamorphic optical element. Moreover, the differential bow can be reduced in the multi-beam scanning optical system.
In the specific embodiment, the first anamorphic optical element comprises a curved surface mirror and the second anamorphic optical element comprises an anamorphic lens. In the case where the reflected light from the curved surface mirror is divergent in the auxiliary scanning direction, the mirror surface of the curved surface mirror is designed so that a power of the mirror surface in the auxiliary scanning direction varies toward a direction to increase a positive power as the distance from a center in the main scanning direction increases. Such a mirror surface may comprise an advanced toric surface that is defined as a locus formed from the rotation of a non-circular curved line extending in the main scanning direction about an axis parallel to the main scanning direction.
When the curved surface mirror has a negative power in the auxiliary scanning direction, an absolute value of a radius of curvature of the curved surface mirror in the auxiliary scanning direction increases gradually as the distance from a center in the main scanning direction increases. When the curved surface mirror has a positive power in the auxiliary scanning direction, an absolute value of a radius of curvature of the curved surface mirror in the auxiliary scanning direction decreases gradually as the distance from a center in the main scanning direction increases.
At least one surface of the anamorphic lens may be formed as an advanced toric surface that is defined as a locus formed from the rotation of a non-circular curved line extending in the main scanning direction about an axis parallel to the main scanning direction. With this construction, since the positive power in the auxiliary scanning direction can be freely determined, it allows to correct the curvature of field in the auxiliary scanning direction. Moreover, the distribution of the power in the auxiliary scanning direction may give a power in the main scanning direction, and thus the lens surface may have a function to correct the curvature of field in the main scanning direction.
Since the advanced toric surface provided in the curved surface mirror and the anamorphic lens is a surface having a rotation axis, a mold for forming the advanced toric surface can be manufactured by a lathe.
Further, the difference of the power of the anamorphic lens in the auxiliary scanning direction between the center and the peripheries along the main scanning direction is determined to counterbalance curvature of field caused by the power variation of the curved surface mirror in the auxiliary scanning direction. The difference of the power of the anamorphic lens is set larger than in the case where the curved surface mirror is not provided with the Power variation in the auxiliary scanning direction.
When a surface of the anamorphic lens is formed as the advanced toric surface, there are two ways to enlarge the difference of the power in the auxiliary scanning direction. One way is to add the power only at the peripheral portion. Another way is to decrease the radius of curvature in the auxiliary scanning direction as a whole. The former way also changes the surface configuration in the main scanning direction and it affects the curvature of field in the main scanning direction and the linearity error. The latter way is, therefore, better than the former way. When the latter way is taken, since the power in the auxiliary scanning direction increases as a whole, the other side surface of the anamorphic lens should have the negative power that counterbalances the additional positive power of the advanced toric surface. As a result, the anamorphic lens is designed so that a positive power thereof in the auxiliary scanning direction decreases gradually from the center to the peripheries along the main scanning direction. Further, one surface of the anamorphic lens has a negative power in the main scanning direction and a positive power in the auxiliary scanning direction, and the other surface of the anamorphic lens has a negative power in the auxiliary scanning direction.
Still further, the anamorphic lens may be designed such that one lens surface thereof is defined as a locus formed by moving the non-circular curved line in the main scanning direction along a non-circular curved line in the auxiliary scanning direction. This type of surface does not have a rotation axis and thus it is difficult to manufacture a mold for forming a lens. However, since the surface can control powers in the main and auxiliary scanning directions independently, it can correct aberrations in both the scanning directions. When one lens surface of the anamorphic lens is formed as the surface having no rotation axis, the other surface may be formed as a simple rotationally symmetrical surface.